Upsampling question - 20x30 inch print resolution?

[amplifier]

Hi


I have an image (4667x7000 @ 300dpi). I want a 20x30 sharp wall print of it. I am going to use photoshop to upsample it. What is the best method for this?
In the Image Size menu in photoshop, should I upsample it at 6000x9000 and keep dpi same at 300 dpi or should I reduce the dpi to 200/150? Which will give sharper image?

 

[The_Traveler]

Don't worry about the dpi, just upsample to the size in pixels you want.

BUT Sharp prints can easily be made with 250 dpi so the extra upsampling (which is just inventing pixels) may not be worth it.

 

[KmH]

If you look closely, in Photoshop it's ppi, not dpi. The 2 terms refer to different things, have different meanings, and are not interchangable.

PPI = pixels per inch
DPI = dots per inch.

Pixels are generally square. Dots are round.
It takes several dots to print 1 pixel.

 

[Garbz]

I have an image (4667x7000 @ 300dpi). I want a 20x30 sharp wall print of it. I am going to use photoshop to upsample it. What is the best method for this?
In the Image Size menu in photoshop, should I upsample it at 6000x9000 and keep dpi same at 300 dpi or should I reduce the dpi to 200/150? Which will give sharper image?

Really for the best results I suggest maybe not using photoshop. Photoshop is surprisingly limited on resampling algorithms. But ultimately it depends on the image. If your picture has lots of smooth gradients then the bicubic resizer that photoshop offers will be just fine. If you're trying to upsample something with lots of detail like a busy city shot or contrasty foliage in a forest then I suggest using something like a Lanczos based resampling algorithm. I think from memory Irfanview is a free program that offers a lanczos interpolator.

BUT Sharp prints can easily be made with 250 dpi so the extra upsampling (which is just inventing pixels) may not be worth it.

There's nothing to lose doing it though. I'd prefer when someone sticks their nose up against the image they see blurry upsampling artefacts than pixels. One looks like a limitation of equipment and the other looks like a crap print. Holding a 250ppi image only a few inches from your face you can see pixels.